Electrical Calculators

How do you calculate watts, amps, volts and ohms?

Use the power formula, P = V × I, for watts and Ohm’s law, V = I × R, for resistance; nearly every everyday electrical calculation comes down to these two relationships. The power formula links watts, volts and amps: P = V × I. Ohm's law links volts, amps and resistance: V = I × R. If you know any two values in either formula, you can rearrange it to find the third.

Rearranging the formulas

• Amps from watts: I = P ÷ V
• Watts from amps: P = V × I
• Volts from amps and ohms: V = I × R
• Amps from volts and ohms: I = V ÷ R

Worked example: a 1,500 W space heater on a 120 V outlet draws 1,500 ÷ 120 = 12.5 A. The same heater on a 230 V supply draws 1,500 ÷ 230 = 6.52 A, which is why appliances in 230 V countries draw roughly half the current for the same power. Its heating element has a resistance of about 120 ÷ 12.5 = 9.6 Ω. Try your own numbers in the watts to amps calculator or its reverse, the amps to watts calculator, or solve any two of V, I and R with the Ohm's law calculator.

When the simple formula is not enough

P = V × I is exact for DC circuits and for resistive AC loads such as heaters, kettles and incandescent lamps. Motors, compressors and many power supplies also have a power factor below 1, so the real power in watts is lower than volts × amps. In that case use P = V × I × PF, or convert apparent power with kW = kVA × PF.

What are volts, amps, watts and ohms?

Volts measure electrical potential difference (the push), amps measure the rate at which charge flows, ohms measure resistance to that flow, and watts measure the rate at which energy is delivered. A garden hose is a useful picture: voltage is like water pressure, current is the flow rate, resistance is how narrow the hose is, and power is how much work the water does each second.

• Ampere (A): one of the seven SI base units. Since 2019 the BIPM SI Brochure defines it by fixing the elementary charge at exactly 1.602176634 × 10⁻¹⁹ coulomb, so 1 A is a flow of about 6.241509 × 10¹⁸ elementary charges per second.
• Volt (V): one watt per ampere (W/A), the potential difference that delivers one joule of energy per coulomb of charge.
• Ohm (Ω): one volt per ampere (V/A), the resistance that lets 1 A flow when 1 V is applied.
• Watt (W): one joule per second. Horsepower and BTU per hour are other units for the same quantity; the power converter handles them.

Household supplies are alternating current (AC), which reverses direction 60 times a second in North America and 50 times a second in most other countries; the frequency converter covers hertz. These quantities are easy to calculate but dangerous to measure or work on at mains voltage: any installation, repair or change to fixed wiring must follow your local electrical code and should be carried out by a licensed electrician.

How do you work out an appliance’s energy use and running cost?

Multiply the power in kilowatts by the hours of use to get kilowatt-hours (kWh), then multiply by the price per kWh shown on your electricity bill. Power is a rate and energy is an amount, and the kilowatt-hour is the amount utilities bill for.

Worked example: a 1,500 W space heater running 4 hours a day uses 1.5 kW × 4 h = 6 kWh per day, or 180 kWh over a 30-day month. A 10 W LED bulb on for 5 hours a day uses 0.01 kW × 5 h × 365 days = 18.25 kWh a year, roughly what that heater uses in 3 days. Multiply either figure by your tariff to get the cost. If an appliance label lists only amps, find the watts first with the amps to watts calculator.

• Nameplate watts are a maximum. Thermostats, compressors and power-saving modes cycle on and off, so real consumption is often lower. A plug-in energy meter measures actual use.
• Standby adds up. Devices that stay plugged in draw a small, constant load around the clock.
• Units of energy. One kWh is exactly 3,600,000 joules; the energy converter translates kWh into joules, BTU or kilocalories.

How much load can a household circuit carry?

A standard 15 A, 120 V circuit can carry up to 1,800 W and a 20 A circuit up to 2,400 W, but loads that run for three hours or more should stay at or below 80% of those figures: 1,440 W and 1,920 W.

• Check circuit capacity before adding a load. A 15 A, 120 V circuit can carry at most 15 × 120 = 1,800 W, and a 20 A circuit 2,400 W. Add up everything that runs at the same time on that circuit.
• Leave headroom for continuous loads. In the US, the National Electrical Code sizes circuits so continuous loads (three hours or more) use no more than 80% of the breaker rating: 12 A (1,440 W at 120 V) on a 15 A circuit, or 16 A (1,920 W) on a 20 A circuit.
• Use nameplate ratings. The label on an appliance lists its watts or amps and voltage. Running power is often lower than the rating, but motors can draw several times their running current for a moment at start-up.
• Wire size follows the breaker, not the other way around. As general guidance for US copper building wire, 14 AWG is typically protected at 15 A, 12 AWG at 20 A and 10 AWG at 30 A. Long runs, high temperatures, bundled cables and aluminum conductors can all require a larger size. Countries that follow IEC-based wiring rules size cable in mm² under their own tables.
• Keep voltage drop small on long runs. A common design target is no more than 3% drop on a branch circuit, which is 3.6 V on a 120 V circuit and 6.9 V on 230 V. Drop grows with current and cable length and shrinks with conductor size, so long runs often need a larger cable than the breaker rating alone suggests.

Safety note: these calculators are for estimating and learning. Electrical installation and repair must follow your local electrical code, such as the NEC in the US or IEC-based national wiring rules elsewhere, and should be carried out by a qualified, licensed electrician.

What is the difference between single-phase and three-phase power?

Single-phase power delivers one alternating voltage and serves most homes, while three-phase power delivers three voltages offset by a third of a cycle (120°) and serves large buildings, workshops and heavy motors.

Homes in North America and much of the world are supplied with single-phase power, typically 120/240 V in North America and 230 V in Europe, the UK, India and Australia. Large buildings, workshops and industrial motors often use three-phase power, which delivers more power with less conductor material and gives motors a smoother, constant torque.

Single-phase vs three-phase formulas

• Single-phase: P = V × I × PF, so I = P ÷ (V × PF).
• Three-phase (line-to-line voltage): P = √3 × V × I × PF, so I = P ÷ (√3 × V × PF), where √3 ≈ 1.732.

For example, a 10 kW three-phase load at 400 V with a power factor of 0.85 draws 10,000 ÷ (1.732 × 400 × 0.85) ≈ 16.98 A per line. For single-phase loads with a power factor of 1, the simpler I = P ÷ V applies. Common three-phase line voltages are 208 V and 480 V in North America and 400 V in Europe.

What power factor means

Power factor is the ratio of real power (kW), which does useful work, to apparent power (kVA), which the supply must deliver. Purely resistive loads have a power factor of 1. Inductive loads such as motors and transformers have a lower power factor, so they draw more current for the same useful power. That is why generators and transformers are rated in kVA: a 10 kVA unit supplying loads at 0.8 PF delivers 8 kW.